Medium to high-power semiconductor lasers are highly desirable in a wide variety of optical applications. More particularly, high-power near-infrared lasers operating at wavelengths ranging from around 760 nm to around 900 nm are desirable for a number of applications such as active night-vision devices and three-dimensional (3D) imaging devices.
Among the various types of lasers available for such applications, one type of laser that is generally referred to in the industry as an edge-emitting Fabry-Perot (FP) laser, can satisfy some requirements pertaining to high power optical output. However, high-power edge-emitting FP lasers are susceptible to catastrophic failures, for example as a result of the high optical power density at the light emitting facet. The manufacturing cost of high-power edge-emitting FP lasers also tends to be high as a result of various post-wafer fabrication processing steps that are difficult to scale up to satisfy high volume manufacturing. Furthermore, the output laser beam of a high-power edge-emitting FP laser can be undesirably divergent and consequently difficult to align to a target object.
Another type of laser that is generally referred to in the industry as a vertical cavity surface emitting laser (VCSEL), addresses some of the shortcomings associated with the edge-emitting FP laser. For example, unlike the edge-emitting FP laser where the optical power density is concentrated over a small area at the light emitting facet, the optical power density in a VCSEL is distributed over a significantly larger lateral surface area thereby providing better device reliability. Also, VCSELs can be manufactured in high volume as a result of various factors such as testability of multiple VCSELs at a wafer level. Such testing is in contrast to manufacture-related testing of edge-emitting FP lasers that can only be carried out upon individual devices after singulation of a semiconductor wafer.
With further reference to VCSELs, VCSELs are typically manufactured in two flavors—“top-emission” VCSELs and “bottom-emission” VCSELs. “Top-emission” VCSELs suffer from various handicaps that hinder their use in high power optical applications. Such handicaps include their inability to satisfy heat dissipation requirements associated for example, with lasing current confinement in an active region of the device.
On the other hand, “bottom-emission” VCSELs can be used in applications where “junction-down” soldering is permissible for more efficient heat-sinking during high-power operation. In this type of VCSEL, light emission takes place through a substrate that constitutes a top surface of the device. However, as a result of light having to propagate through the substrate, “bottom-emission” VCSELs are typically limited to operating over certain wavelengths at which light can propagate through the material of the substrate. In other words, the substrate material has to be “transparent” to these optical wavelengths and such a requirement places a limitation upon useable optical wavelengths.
In summary, in view of the remarks above, it is desirable that various shortcomings related to traditional lasers and particularly to VCSELs, be addressed.